1. Field of the Invention
The present invention relates to a fluorescent display device which emits blue or green light under low-velocity electron excitation, and more particularly to a fluorescent display device comprising a vacuum tube including therein an anodic plate bearing a fluorescent screen made of a novel fluorescent material and a cathode to emit low-velocity electrons.
2. Description of the Prior Art
There is well known a low-velocity electron excited fluorescent display device (hereinafter referred to simply as "display device" or "fluorescent display device") essentially comprising a vacuum tube having disposed therein an anodic plate having on one surface thereof a fluorescent screen and a cathode as an electron source facing said fluorescent screen which screen is caused to emit luminescence when low-velocity electrons emitted from the cathode impinge thereon.
As a phosphor capable of providing high luminescence upon excitation by low-velocity electrons, zinc-activated zinc oxide phosphor (ZnO:Zn) has hitherto been known. The phosphor may be prepared by firing zinc oxide in a reducing atmosphere or by firing in air zinc oxide added with a small amount of a zinc compound such as zinc sulfide. This phosphor emits green-white light of high luminance when excited by low-velocity electrons.
The display tubes having a fluorescent screen of ZnO:Zn are now been widely used as display devices for desk or pocket electronic calculators, various measuring instruments and so forth. However, the color of the light emitted from ZnO:Zn under the low-velocity electron excitation is greenish white and hence the aforesaid conventional display devices using ZnO:Zn as the fluorescent screen show insufficient color purity. Therefore, a fluorescent display device capable of emitting green light of excellent color purity is desired.